1. Field of the Invention
This invention relates to a zoom lens system having chiefly so-called organic glass lenses known as plastic lenses.
2. Description of the Prior Art
In various lens systems, if all lenses as constitutional elements are replaced by by organic glass lenses, called the plastic lenses, the weight of the system will be reduced by 1/3-1/2 and by virtue of this great reduction in weight manufacturing cost can be brought about. However, plastics have large expansion coefficients and also have large variations of refractive index with temperature, as compared with inorganic glass, and this has led to the disadvantage that the focal plane of the lens is moved by any variation in temperature.
To overcome such disadvantage, in a fixed focal length lens of triplet construction, a technique of forming one of positive lenses of glass, another positive lens of plastic and a negative lens of high dispersion plastic and thereby negating any temperature variation while correcting chromatic aberration is known from Japanese Laid-open Patent Application No. 143518/1980.
In this Japanese Laid-open Patent Application No. 143518/1980, for a fixed focus lens system comprising N lenses including a plastic lens, when the total focal length of the lens system is f, the ith lens is determined as Li, the focal length of the lens Li at a standard temperature t is f.sub.i, the height of incidence on the lens Li of a paraxial ray incident from the object side at a height f is hi, the refractive index of the lens Li at the temperature t is n.sub.i (t), and the thermal dispersion number .omega..sub.i of the material forming the lens Li is defined as ##EQU1## is the "condition of athermalization". This condition means that the value of the sum of the thermal aberration coefficients of the respective lenses over the entire system is zero. By this, the amount of movement of the focus by temperature with respect to the two points, i.e., temperatures t.sub.1 and t.sub.2, becomes zero.
Strictly, however, this is limited to a fixed focal length lens and in a zoom lens, the focus position varies greatly between the shortest focal length condition and the longest focal length condition. For example, when zooming is effected into the shortest focal length condition with the focus adjusted in the longest focal length condition, in-focus is attained at normal temperature but out-of-focus is brought about at high temperatures or at low temperatures. This drawback is a more serious drawback than in the case of a fixed focal length lens in which the distance scale goes wrong. For this reason, in the field of zoom lens system comprised of plastic lenses, zoom lenses of low magnification in which the movement of the focus is negligible are only found in optical systems for projection as described in U.S. Pat. Nos. 3,972,592 and 3,920,315, and the technique of correcting the focus position fluctuation resulting from any temperature change, namely, the so-called athermalization technique, remains unexplored in this field.